[R-pkg-devel] Too many cores used in examples (not caused by data.table)

[Dirk Eddelbuettel]

I have some better news.  While we established that 'in theory' setting the
environment variable OMP_NUM_THREADS would help (and I maintain that it is a
great PITA that CRAN does not do so as a general fix for this issue) it does
*not help* once R is started.  OpenMP only considers the variable once at
startup and does not re-read it.  So we cannot set from R once R has started.

But OpenMP offers a setter (and a getter) for the thread count value.

And using it addresses the issue.  I created a demo package [1] which, when
running on a system with both OpenMP and 'enough cores' (any modern machine
will do) exhibits the warning from R CMD check --as-cran with timing enabled
(i.e. env vars set).  When an additional environment variable 'SHOWME' is set
to 'yes', it successfully throttles via the exposed OpenMP setter.  In our
example, Armadillo uses it to calibrate its thread use, a lower setting is
followed, and the warning is gone.

I will add more convenient wrappers to RcppArmadillo itself. These are
currently in a branch [2] and their use is illustrated in the help page and
example of fastLm demo function [3].  I plan to make a new RcppArmadillo
release with this change in the coming days, the setter and re-setter will
work for any OpenMP threading changes. So if you use RcppArmadillo, this
should help. (And of course there always was RhpcBLASctl doing this too.)

Dirk

[1] https://github.com/eddelbuettel/rcpparmadilloopenmpex
[2] https://github.com/RcppCore/RcppArmadillo/tree/feature/thread_throttle\
[3] https://github.com/RcppCore/RcppArmadillo/blob/a8db424bd6aaeda2ceb897142d3c366d9c6591c7/man/fastLm.Rd#L72-L98
[4] https://cran.r-project.org/package=RhpcBLASctl

-- 
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